1. Field of the Invention
The present invention relates to stabilization of a characteristic of an analog circuit formed on a semiconductor chip.
2. Description of the Related Art
Presently, a semiconductor chip is composed of an analog circuit and a digital circuit in most cases. An analog circuit is a generic name of a circuit that uses a signal of an analog value as data and, for instance, includes a standard voltage generation circuit, a phase locked loop (PLL), an analog-digital converter circuit, a digital-analog converter circuit, a phase comparison circuit, etc. A digital circuit is a generic name of a circuit that uses a signal of a digital value as data and includes an arithmetic circuit, which is composed of various logical circuits, and memories.
Generally, a digital circuit is immune to noise as the circuit handles digital signals, and processes at high speed and is low power consumption as the circuit can be operated by faint signals. On the other hand, an analog circuit is a circuit of which an operational characteristic is volatile due to power-supply voltage, temperature and manufacturing variations as the analog circuit handles analog signals. Therefore, a characteristic of an analog circuit may be adjusted by a nonvolatile register composed of a fuse circuit and EEPROM.
FIG. 1 shows a plan view of a conventional semiconductor chip package, and FIG. 2 shows a cross sectional view of a semiconductor chip package taken along the line C-C of FIG. 1. A digital circuit 102 and an analog circuit 103 are formed on a semiconductor chip 101. A nonvolatile register 104 is also formed on the semiconductor chip 101 to store information to adjust a circuit characteristic of the analog circuit 103. The circuit characteristic of the analog circuit 103 is adjusted according to the information stored in this nonvolatile register 104. The digital circuit 102, the analog circuit 103 and the nonvolatile register 104 are coated with insulation film.
The semiconductor chip 101 also has a pad 107 to transfer signals between these digital circuit 102 and analog circuit 103, and the outside of the semiconductor chip on the periphery. The pad 107 is an electrode that bonds a wire 109 to a lead frame 108 (not shown in FIG. 1).
FIG. 3 is an exemplification of the analog circuit 103 and the nonvolatile register 104 of the semiconductor chip 101. In the analog circuit 103, a voltage Vin generated by a bandgap reference circuit (BGR) 111 is input to a plus input terminal of an operational amplifier 112, and an output voltage Vout is output from the operational amplifier 112. Resistances R1, R2, R3, R4 and R5 are connected in series between the output voltage Vout and a ground voltage GND. A terminal voltage between the resistance R1 and the resistance R2 is input to a minus input terminal of the operational amplifier 112 through a switch S1. In a similar way, a terminal voltage between the resistance R2 and the resistance R3, a terminal voltage between the resistance R3 and the resistance R4 and a terminal voltage between the resistance R4 and the resistance R5 are input to the minus input terminal of the operational amplifier 112 through switches S2, S3 and S4 respectively. Only one of the switches S1, S2, S3 and S4 is turned on and three other switches are turned off according to the information stored in the nonvolatile register 104.
Here, the formula, Rtotal=R1+R2+R3+R4+R5, is used. For instance, when the switch S1 is turned on, an output voltage will be Vout=Vin×Rtotal/(R2+R3+R4+R5). In a similar way, when the switch S2 is turned on, an output voltage will be Vout=Vin×Rtotal/(R3+R4+R5). When the switch S3 is turned on, an output voltage will be Vout=Vin×Rtotal/(R4+R5). When the switch S4 is turned on, an output voltage will be Vout=Vin×Rtotal/(R5). Thus, the output voltage Vout, which is the circuit characteristic of the analog circuit 103, is adjusted by selecting which one of the switches S1, S2, S3 and S4 to be turned on.
The nonvolatile register 104 is composed of a fuse circuit that stores information depending on the presence or absence of cuttings of fuse element by laser irradiation and a part that stores information in a nonvolatile memory such as EEPROM. The nonvolatile register 104 outputs a signal that turns on only one of the switches S1, S2, S3 and S4 according to the information stored.
In a production process of the semiconductor chip 101 exemplified in FIG. 1, it is appropriate to screen defects in the circuit characteristic of the analog circuit 103 at the early stage. Then, it is judged whether the circuit characteristic of the analog circuit 103 is in an allowable range depending on the information stored in the nonvolatile register 104 at the stage of a so-called wafer sort with the semiconductor chip 101 formed on a wafer.
However, it is found that the circuit characteristic adjusted at the stage of wafer sort changes when the semiconductor chip 101 of which the circuit characteristic of the analog circuit 103 is adjusted at the stage of the wafer sort as stated above is packaged and sealed with mold resin.
This invention is achieved in view of the above situation and intends to appropriately suppress a change, which is caused by packaging, of a circuit characteristic of an analog circuit of which characteristic is adjusted by a nonvolatile register.
In addition, this invention also intends to appropriately suppress a change, which is caused by packaging, of a circuit characteristic of an analog circuit even in the case that a circuit characteristic of an analog circuit is not adjusted according to information stored in a nonvolatile register.